Recently, tremendous research efforts have been directed toward various display modes using smectic phases in addition to research efforts toward liquid crystal elements using the nematic phase, and in particular surface-stabilized ferroelectric liquid crystal elements have been considered highly prospective since they have superior features, such as a wide angle of view, fast response and bistability, and are applicable to passive-matrix-type displays with large display capacities. The surface-stabilized ferroelectric liquid crystal elements are disclosed in, for example, Appl. Phys. Lett., 36,899 (1980) by N. A. Clark et al.
The passive-matrix-type displays have a construction wherein an insulating substrate having a plurality of scanning electrodes that are formed in parallel with one another and an insulating substrate having a plurality of signal electrodes that are formed in parallel with one another are placed so that the scanning electrodes and the signal electrodes orthogonally intersect each other, and a liquid crystal is sandwiched between the insulating substrates. Regions at which the scanning electrodes and the signal electrodes intersect each other form respective pixels. Thus, the difference voltage between a voltage applied to each of the scanning electrodes and a voltage applied to each of the signal electrodes is applied to each of the pixels.
During a period when, upon driving, a certain scanning electrode is selected, a selection-voltage waveform is applied to the scanning electrode, and signal-voltage waveforms, which correspond to states to be displayed on the respective pixels on the scanning electrode, are applied to the signal electrodes that correspond to the respective electrodes. In this manner, the scanning electrodes are successively selected while at the same time the pixels on the respective scanning electrodes are allowed to have desired display states, thereby forming a display of one frame. Therefore, the signal-voltage waveforms that correspond to the display states on the scanning electrode that is currently being selected are also applied to respective pixels on scanning electrodes that are not selected; however, a driving waveform which prevents these signal-voltage waveforms from changing the display state is adopted.
In the case of the surface-stabilized ferroelectric liquid crystal element, only two states of display, "bright" and "dark", are basically displayed because of its bistability. Therefore, with respect to signal-voltage waveforms for driving the surface-stabilized ferroelectric liquid crystal element, only two types of waveforms are necessary; a signal-voltage waveform that instructs "rewriting" for making a switchover from one stable state to the other stable state and a signal-voltage waveform that instructs "non-rewriting" for retaining the stable state that was held immediately before. Therefore, the driving operation of the surface-stabilized ferroelectric liquid crystal element is carried out by using a driving-voltage waveform that satisfies the following conditions:
(1) the display state should be rewritten by a rewriting-voltage waveform that is applied to a pixel by the combination of the signal-voltage waveform that instructs "rewriting" and the selection-voltage waveform, PA0 (2) the display state should not be rewritten by a non-rewriting-voltage waveform that is applied to a pixel by the combination of the signal-voltage waveform that instructs "non-rewriting" and the selection-voltage waveform, PA0 (3) during the non-selection period, the display state should not be rewritten by any of the states "rewriting" and "non-rewriting" of the signal-voltage waveforms.
In general, whichever display state, "rewriting" or "non-rewriting", the voltage waveform to be applied to a pixel may instruct, the display state is maintained when the voltage or pulse width within the voltage waveform does not reach a threshold value, and the display state is rewritten when both the voltage and pulse width exceed the respective threshold values.
Actually, since the threshold value has variations to a certain extent for each minute region of a pixel, a driving margin ranges from not less than a threshold value for completely switching all the region of a pixel by using the rewriting-voltage waveform to not more than a threshold value for completely retaining the display state of all the region of a pixel by using the non-rewriting-voltage waveform. The driving margin is greatly dependent on the difference between the rewriting-voltage waveform and the non-rewriting-voltage waveform, that is, the degree of the mutual difference between the signal-voltage waveforms that instruct "rewriting" and "non-rewriting".
Various properties including response speed, etc. in liquid crystal materials used for ferroelectric liquid crystal elements are greatly dependent on temperature, and the driving margin also varies depending on temperature changes in service environments. In other words, if the driving margin is narrow, the temperature range in which a liquid crystal element is operable also becomes narrow; therefore, it is necessary to provide an element and a driving method by which a wider driving margin is obtained.
Moreover, in the case of formation of a display element with a large display capacity by utilizing a high-speed response which is one of the features of a ferroelectric liquid crystal element, a problem of heat generation of the element is raised since a high-frequency voltage is required as the driving voltage. This is mainly because upon application of a high-frequency voltage, charging and discharging with high frequencies are carried out on the liquid crystal that is a dielectric so that the electrode lines through which the charging and discharging currents flow generate heat.
For example, in the case when a writing operation is carried out on 2000 scanning lines at 60 Hz per 1 frame by using a driving waveform whose unit pulse width constituting a driving-voltage waveform is a 1/2 of the selection period, a writing period of time per scanning line comes to 8.3 .mu.s so that a signal-voltage waveform is applied to the signal electrode lines with a frequency of as high as 120 kHz. Moreover, in the case of gray-scale display using time division, a further high- frequency operation is of course carried out depending on the number of gradations.
The quantity of heat generation of an element due to the application of a high-frequency voltage resulted from utilization of such a high-speed response is dependent on factors, such as the frequency and voltage value of a signal-voltage, the electrostatic capacitance of the element and the value of the electrode resistance. One method for easily reducing the heat generation is to reduce the signal voltage; however, this method reduces the mutual difference between the signal-voltage waveforms instructing "rewriting" and "non-rewriting", thereby failing to provide a preferable method since the driving margin is narrowed.
Furthermore, in order to achieve a ferroelectric liquid crystal element with such a large display capacity, a ferroelectric liquid crystal material allowing for a high-speed response is required. For this purpose, it is necessary to reduce the viscosity of the ferroelectric liquid crystal material or to increase the spontaneous polarization thereof. However, the excessive increase of spontaneous polarization causes problems, such as malswitching, thereby making it difficult for the ferroelectric liquid crystal material to be applied to display elements.